The present invention relates generally to methods for the conversion of fullerenes to diamond or diamond-like films. More particularly, the invention is concerned with the manufacture of nonhydrogenic fullerenes as precursors for the synthesis of diamond or diamond-like films. Because of their thermodynamic instability with respect to diamond, the conversion of fullereries to diamond has significant advantages over presently used methods for producing synthetic diamonds.
The prior art methods of manufacturing diamond can be divided into two main approaches. In the first or high-pressure method, graphite powder is heated to about 1500.degree. C. under 60 kbar of pressure in the presence of an iron catalyst. Under this extreme, but equilibrium, condition of pressure and temperature, graphite can be converted to diamond. About 75 tons of diamond are produced industrially each year in this way. The second or low pressure method of producing diamond artificially involves producing a mixture usually of a few percent of methane in hydrogen gas. A hot filament or a microwave discharge is used to dissociate the methane molecule to form the methyl radical, CH.sub.3, and the hydrogen molecule is dissociated to form hydrogen atoms. Diamond or diamond-like films can be grown this way epitaxially on diamond nuclei. Such films, however, always contain small contaminating amounts (0.1-1%) of hydrogen which are undesirable for many applications.
The usefulness and novelty of fullerene precursors for diamond synthesis stem from several of their properties: Fullerene precursors are thermodynamically unstable with respect to diamond and, therefore, stable only in a kinetic sense. In addition, since the fullerenes are molecular entities, they are volatile with C.sub.60 having a vapor pressure of 10.sup.-4 Torr at 500.degree. C. Fullerenes are also allotropes of carbon; that is, they contain no hydrogen; and therefore, diamonds produced from fullerene precursors are hydrogen-defect free. Another useful characteristic of fullerene is the chemical bond in C.sub.60 is intermediate between graphite (sp.sup.2) and diamond (sp.sup.3). Furthermore, fragmentation involving carbon-carbon bond breakage occurs via the elimination of C.sub.2 groups. Recent scanning tunneling microscope studies have shown C.sub.2 groups to be intimately involved in the growth of epitaxial diamond films, particularly of the "dimer rows" of the 2.times.1 reconstructed &lt;100&gt; surface. It has also been determined both the positive C.sub.60.sup.+ and negative C.sub.60.sup.- ions are stable entities that can be accelerated to kilovolt energies under an applied electrostatic potential. The so-called LUMO (lowest unoccupied molecular orbital) of the fullerenes also is an antibonding three-fold degenerate orbital that can accept up to six electrons from electron donors such as the alkali metals. The resultant repulsion between delocalized electrons weakens the carbon-carbon bonds of the C.sub.60 cage and provides a mechanism for the fullerene to undergo diamond transformation.
In particular, fragmentation of fullerenes involving carbon-carbon bond breakage can be achieved by energizing the fullerene molecule in a variety of ways including acceleration of the ionized molecule with subsequent bombardment of a surface resulting in diamond-like film growth. The variety of methods for energizing fullerene molecules with the objective of fragmentation or incipient fragmentation includes, but is not limited to, any conventional means of inducing molecular fragmentation such as by absorption of photons; fragmentation by passage over a hot filament; fragmentation in a radio frequency or KF-plasma; fragmentation in a DC plasma jet; and fragmentation in a microwave plasma.
In all of these cases, fragmentation of fullerenes (as for fragmentation of any molecule) occurs in part as a result of absorption of internal energy in the form of rotational, vibrational, and electronic energy. The rate of fragmentation depends on the degree of energy absorption and can become appreciable when the fullerene molecules become highly energized, corresponding to the absorption of 40-50 eV of internal energy which in turn corresponds to a thermal "temperature" of 3000.degree.-4000.degree. K. A fullerene molecular ion, which has acquired a high state of internal energy, consequently needs only a small acceleration potential (10-100 V) to induce fragmentation on collision with a surface. Such an acceleration potential can be supplied either by the plasma potential itself or by DC or RF biasing of the substrate on which diamond or diamond-like film growth takes place.
The mechanisms by means of which the fullerene molecules acquire their high internal energies, which are a prerequisite to molecular fragmentation in the absence of a large (.about.1 keV) external acceleration potential, are, for example, the conventional molecular fragmentation method of (1) photon absorption in the case of photon irradiation leading to photofragmentation, (2) thermal energy absorption in the case of passage of fullerene molecules over a hot filament, (3) electron and ion collisions in the case of RF, DC, or microwave plasmas. It is well known to anyone versed in the art that imparting sufficient energy to hydrocarbon molecules such as methane, acetylene, benzene, etc. and to other species such as graphite results in fragmentation, that is to say, carbon-carbon and carbon-hydrogen bond breaking. Such fragmentation, leading to the production of methyl radicals, CH.sub.3 is a key step for diamond film formation from methane, for example. A variety of methods for imparting sufficient energy to hydrocarbon molecules so as to fragment them with subsequent formation of diamond films has been devised and successfully used in a now conventional manner. Among these methods are a hot filament, an RF plasma discharge device, a DC-thermoplasma device, a DC-plasma jet device, a high intensity photon source and a microwave plasma.
In the case of fullerenes, energy supplied by heat, photons, electrons, or collisions with gas molecules likewise results in fragmentation with the production of C.sub.2 fragments. The devices to supply fragmentation energy referred to above are well known to impart energy to molecules.
All of the methods for imparting internal energy to fullerene molecules also lead to varying degrees of ionization as a result of photon absorption as well as electron and ion collisions. Electron emission from C.sub.60 results in C.sub.60.sup.+ while electron attachment gives C.sub.60.sup.-. Depending on the magnitude of energy absorbed by the fullerene molecules, fragmentation occurs either in the gas phase or, with the help of a small (10-100 eV) bias potential, on collision with a substrate.